The United States Government has rights in this invention pursuant to the employer-employee relationship of between the U.S. Department of Energy and the inventors.
This invention relates generally to electrochemical devices and is particularly directed to improvements in the process gas distribution and available active surface area in a solid state fuel cell.
Fuel cells are electrochemical systems that generate electrical current by chemically reacting a fuel gas and an oxidant gas on the surface of electrodes. Conventionally, the components of a single fuel cell include the anode, the cathode, the electrolyte, and the interconnect material. In a solid state fuel cell, such as solid oxide fuel cells (SOFCs), the electrolyte is in a solid form and insulates the cathode and anode one from the other with respect to electron flow, while permitting oxygen ions to flow from the cathode to the anode, and the interconnect material electronically connects the anode of one cell with the cathode of an adjacent cell, in series, to generate a useful voltage from an assembled fuel cell stack. The SOFC process gases, which include natural or synthetic fuel gas (i.e., those containing hydrogen, carbon monoxide or methane) and an oxidant (i.e., oxygen or air), react on the active electrode surfaces of the cell to produce electrical energy, water vapor and heat.
Several configurations for solid state fuel cells (i.e., fuel cells having a solid electrolyte) have been developed, including the tubular, flat plate, and monolithic designs. In a tubular design, each single fuel cell includes electrode and electrolyte layers applied to the periphery of a porous support tube. While the inner cathode layer completely surrounds the interior of the support tube, the solid electrolyte and outer anode layers are discontinuous to provide a space for the electrical interconnection of the single fuel cell to the exterior surface of adjacent, parallel cells. Fuel gas is directed over the exterior of the tubular cells, and oxidant gas is directed through the interior of the tubular cells.
The flat plate design incorporates the use of electrolyte sheets which are coated on opposite sides with layers of anode and cathode material. Ribbed distributors may also be provided on the opposite sides of the coated electrolyte sheet to form flow channels for the reactant gases. A conventional cross flow pattern is constructed when the flow channels on the anode side of the electrolyte are perpendicular to those on the cathode side. Cross flow patterns, as opposed to co-flow patterns where the flow channels for the fuel gas and oxidant gas are parallel, allow for simpler, more conventional manifolds to be incorporated into the fuel cell structure. A manifold system delivers the reactant gases to the assembled fuel cell. The coated electrolyte sheets and distributors of the flat plate design are tightly stacked between current conducting bipolar plates. In an alternate flat plate design, uncoated electrolyte sheets are stacked between porous plates of anode, cathode, and interconnecting material, with gas delivery tubes extending through the structure.
The monolithic solid oxide fuel cell (MSOFC) design is characterized by a honeycomb construction that is fused together into a continuous structure. The MSOFC is constructed by tape casting or calendar rolling the sheet components of the cell, which include thin composites of anode-electrolyte-cathode (A/E/C) material and anode-interconnect-cathode (A/I/C) material. The sheet components are corrugated to form co-flow channels, wherein the fluid gas flows through channels formed by the anode layers, and the oxidant gas flows through parallel channels formed by the cathode layers. The monolithic structure, comprising many single cell layers, is assembled in a green or unfired state and co-sintered to fuse the materials into a rigid, dimensionally stable SOFC core.
These conventional designs have been improved upon in the prior art to achieve higher power densities. Power density is increased by incorporating smaller single unit cell heights and shorter cell-to-cell electronic conduction paths. SOFC designs have thus incorporated thin components which are fused together to form a continuous, bonded structure. However, the large number of small components, layers, and interconnections, in addition to complex fabrication steps, decreases the reliability of operational fuel cells. In addition, any given fuel cell design must be commercially viable as an alternative power generating device, and, therefore, factors affecting the economics of power generation by electrochemical activity, such as overall capital and operational costs to the user, must be comparable to those of conventional power generating systems.
The present invention is directed to improving the process gas distribution and available active surface area in a solid state fuel cell having a unique planar tube-sheet design. Accordingly, the fuel cell stack is constructed from individual planar sheets of integrally connected, parallel tubes. The fuel cell stack is assembled by stacking the individual planar tube-sheets, such that the tubes within each sheet conduct a first process gas horizontally through the fuel cell stack, and spaces formed between adjacent stacked sheets define gas flow passages for conducting a second process gas horizontally through the fuel cell stack. With respect to each individual sheet, a series of perforations are formed in the sheet material between and connecting adjacent tubes. The perforations, referred to herein as xe2x80x9clateral fuel aperturesxe2x80x9d due to their disposition on the sheet material extending to the side of each tube, importantly conduct the second process gas vertically through the fuel cell stack.
Advantageously, the lateral fuel apertures result in significantly improved fuel distribution within the fuel cell stack, such that the occurrence of harmful hot spots is substantially reduced. The lateral fuel apertures also provide a greater active cathode-electrolyte-anode surface area within the fuel cell stack for achieving higher power densities, as well as anode-electrolyte continuity about the circumference (from the top to the bottom) of each tube, representing a low cost method for maximizing fuel cell active surface area. This solid state fuel cell design is a viable technology for future commercial installations.
Therefore, an object of the present invention is to provide a solid state fuel cell design incorporating unique lateral fuel apertures that improves fuel distribution and substantially eliminates the formation of hot spots within the fuel cell assembly.
Another object of the present invention is to provide a solid state fuel cell design incorporating unique lateral fuel apertures that increase the active surface area per unit fuel cell, such that the overall power density of the assembled fuel cell stack is critically improved.
Another object of the present invention is to simplify the construction of an assembled fuel cell system by forming and stacking planar sheets of integrally connected tubes, preferably manufactured by a single extrusion step.
Yet another object of the present invention is to increase current flow within the fuel cell system by graduating the thicknesses of the electrode structures of the planar sheets of integrally connected tubes, according to the direction of the current flow through the fuel cell stack.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of instrumentation and combinations particularly pointed out in the appended claims.
Briefly, this invention is a solid state electrochemical device that incorporates lateral fuel apertures into a monolithic fuel cell assembly constructed from stacking planar sheets of integrally connected, parallel tubes. The design significantly improves fuel distribution within the fuel cell assembly and increases the available active surface area per unit fuel cell to achieve greater power densities.
Individual planar sheets are composed of a series of parallel, longitudinally aligned tubes that are integrally connected along their lengths to define the sheet. Importantly, adjacent tubes within the sheet are spaced a desired distance apart and connected one to the other by continuous, laterally extending sheet material. The individual planar sheets of integrally connected tubes are preferably fabricated from cathode material and easily and economically manufactured by a single extrusion step. The tubes have open ends for receiving and discharging an electrochemical process gas. A layer of electrolyte material is applied to the external surfaces of the cathode body (interconnect strip excepted, as explained below), and the electrolyte surface is then coated by an anode material, resulting in an active cathode-electrolyte-anode composite surface. The top surface layers of electrolyte and anode material are interrupted by interconnect strips that are applied to the top surface of the planar sheets, along the length of each tube.
The solid state electrochemical device is assembled by uniformly stacking the individual planar sheets (all tubes are parallel), such that points of contact between adjacent sheets is limited to the interconnect strip of a lower sheet contacting and supporting the anode layer of an upper sheet. In operation, the tubes define oxidant gas flow passages extending horizontally through the assembled fuel cell stack. Longitudinal passages formed between adjacent, stacked planar sheets define fuel gas flow passages extending horizontally through the assembled fuel cell stack.
A critical feature of the invention are lateral fuel apertures passing through the connecting material between adjacent tubes within each planar sheet. The lateral fuel apertures are preferably a row of perforations configured to conduct fuel gas vertically through the stacked array of planar sheets, improving the uniformity of fuel distribution throughout the assembly, such that damaging hot spots caused by uneven or mal-distribution of the fuel is avoided. The lateral fuel apertures connect a lower horizontal gas flow passage formed between adjacent sheets with an upper horizontal gas flow passage formed between adjacent sheets. The lateral fuel apertures may be staggered on either side of a tube to effectively manage mechanical stress within the structure of the fuel cell stack. In addition, the interior cathode surfaces of the lateral fuel apertures are coated with electrolyte and anode layers, to provide for additional cathode-electrolyte-anode active surfaces within the fuel cell stack.
Another feature of the invention is the variation of the thicknesses of the cathode body and the anode layers about the circumference of the tubes within the planar sheets, according to the direction of current flow downwardly through the fuel cell stack. Varying the thickness of the electrode materials reduces resistance of the current path.